This invention relates to oxidation or converter apparatus for removing volatile organic compounds from an air stream.
An increasing problem encountered by industry, and particularly fuel supply companies, is the contamination of the air by volatile organic compounds (VOC). Frequently contamination problems are encountered due to accidental spilling of these compounds onto the ground or by leakage from equipment or storage facilities, typically underground tanks in which these compounds are stored. Because of increasing environmental concerns, governments have been imposing stricter standards on industry both to prevent the contamination of the environment, including the ground, and to require a thorough clean up of any contamination that occurs. In order to determine the extent of ground contamination, test bores can be drilled into the affected front. Once the extent of contamination has been determined, there are several known methods of removing the contamination. The commonly used methods have some drawbacks. One way for soil remediation is to excavate the contaminated soil and dispose of it in an approved hazardous solid waste dump site but the cost of this solution is very high. Furthermore, there can be a revenue loss to the property owner due to the need to shut down the facility in question during excavation.
Another method to remove contamination from soil is to use a vapor extraction system. This requires bore holes to be formed in the contaminated ground so as to permit air ladened with volatile organic compound (VOC) to be collected. In this method, underground interconnecting PVC pipes and vacuum pumps can employed to collect the contaminated air, which air is then pumped through an activated carbon adsorption bed, a thermal converter or a catalytic converter. The use of an activated carbon adsorption system has distinct disadvantages from the standpoint of the carbon and the need to dispose of contaminated carbon at an approved solid waste dump site. An expensive monitoring service may be required to collect monitoring data for evaluation in order to comply with environmental standards.
The problem with using only a thermal converter to cleanse the contaminated air is the high cost due to high energy consumption and the need for a monitoring service. The latter is required to collect monitoring data for evaluation to comply with environmental standards. Energy costs can be high because of the need to supply a relatively expensive, clean burning fuel to the combustion chamber when the volatile organic compounds in the contaminated air falls below a certain level. A catalytic converter suffers from the difficulty that it is not suitable where the contaminated air contains high levels of VOC compounds, that is, where the low explosive limit (LEL) of the contaminated air is too high.
U.S. Pat. No. 4,983,364 issued Jan. 8, 1991 to F. A. Buck et al describes a combination thermal/catalytic combustor for the oxidation of VOC compounds in air, which apparatus is said to be suitable for cleaning up ground sites contaminated by spills or leaks. A thermal combustor and a catalytic oxidizer are interconnected by piping and control valves and when one operates, the other does not. The selection depends on the concentration of combustible vapors in the air. This patent specification also teaches the use of a dilution air valve, a heat exchanger, and an electric heater to improve the efficiency of the system. The heat exchanger is heated on one side by exhaust air exiting from the outlet of the catalytic oxidizer. This known multi-mode combustor has several disadvantages including the need for extensive monitoring on site and manual operation of many of the valves in the piping system. Also the use of an electric heater for heating the air entering the catalytic converter can increase substantially the cost of operation of this combuster.
The present invention provides an improved oxidation or converter apparatus for cleansing air contaminated by VOC compounds by means of an exothermic reaction. The apparatus can employ both a combustion chamber with a burner and a catalytic oxidizer, both having inlets for VOC contaminated air and outlets to atmosphere for exhaust air that can include carbon dioxide and water. Advantageous features that can be employed in the oxidation apparatus described herein include piping means for routing the VOC contaminated air through the combustion chamber in order to heat same before it reaches the catalytic oxidizer and a heat exchanger which can also be used to heat the VOC contaminated air before it reaches the combustion chamber and then the catalytic oxidizer. Further advantageous features disclosed herein include a special condensation tank for removing condensation or water vapor from the VOC contaminated air as it enters the converter system and a mechanism for introducing a selected amount of fresh air into the entry pipe so that the fresh air is mixed with the VOC contaminated air. The damper mechanism for controlling the entry of fresh air is operated by an electrical control which includes means for measuring the low explosive limit (LEL) of the VOC contaminated air in the entry pipe.